Based on a set of transient simulations over the past millennium, this study applies multivariate empirical orthogonal function (MVEOF) analysis to investigate the interannual relationship between the East Asian subtropical westerly jet (EASWJ) and the East Asian summer monsoon (EASM) and its underlying mechanisms during three periods of the past millennium: the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the present warm period (PWP). The results show that, on the interannual time scale, the first MVEOF mode during all three periods consistently represents the relationship between the northwest–southeast displacement of the EASWJ and the intensity of the EASM, which is attributable to the internal variability of sea surface temperature (SST) over the Pacific, Indian, and Atlantic Oceans. The second MVEOF mode reveals that during the MCA and LIA, the dominant feature is an out-of-phase (in-phase) relationship between EASWJ and EASM intensities in the midlatitudes (subtropics), whereas during the PWP it is characterized by a zonal displacement of the EASWJ, with an eastward (westward) shift associated with a stronger (weaker) EASM. The intensity relationship during the MCA and LIA is modulated by solar and volcanic activities, respectively. The SSTs over the three oceans jointly modulate the relationship between the EASWJ position and the EASM intensity. The Victoria Mode (VM) mainly affects the EASWJ–EASM intensity relationship. The VM together with the Indian Ocean Basin Mode regulates the relationship between the EASWJ zonal displacement and the EASM intensity. 本研究利用CESM-LME数据, 结合多元经验正交函数(MVEOF)分析方法, 研究了过去千年三个特征时期(MCA,LIA,PWP)东亚副热带西风急流(EASWJ)与东亚夏季风(EASM)在年际尺度上的协同变化关系以及对应机制.结果表明, 第一主导模态反映的是由三大洋海温内部变率调控的EASWJ西北-东南向位移与EASM强度之间的关系, 该模态在所有三个时期均保持一致;而第二主导模态则呈现明显的模态转变—在MCA和LIA时期, 该模态主要表现为受自然外强迫调控的强度-强度关系, 但在PWP转变为由太平洋和印度洋海温内部变率调控的EASWJ纬向位移-EASM强度关系模态.本研究对比了内部变率与自然,人为外强迫对东亚气候的差异化调控机制, 可为全球变暖背景下的未来气候预估提供参考.
Fan et al. (Sun,) studied this question.